Santorum also said that with GOP presidential candidate Sen. Ted Cruz (R-TX), “you’ve seen a shift” on some issues.

Breitbart News asked Santorum if he believes Trump is sincere on his conservative principles following the recent criticism by Cruz about Trump’s “New York values” shared in a 1999 interview between Trump and Tim Russert.

“Past is prologue and if you look at someone’s past, it’s legit,” Santorum began, but added, “people change and that’s a good thing. I want to encourage everybody to be more conservative.”

“You look at some of the greatest conservatives, many of them changed positions over the years,” Santorum added. He said:

People do change their minds over 15-year period of time. I mean, if it’s something you did last year or the year before, then I’m thinking, ‘Well, maybe you’re conveniently changing your mind for the purposes of running for office.’ I think if you go back 15 years and you’ve seen a progression, I’m not sure that’s legit.

“I went to visit Donald Trump about a year an a half ago in his office when I was in New York and it was about three months after I wrote my book Blue Collar Conservatives,” Santorum explained. “He was holding a copy of my book.”

GOP presidential candidate former Arkansas Gov. Mike Huckabee also defended Trump in an interview on Saturday against Cruz’s criticism.

Santorum took a more skeptical line when asked about Cruz.

“You’ve seen a shift,” he said.

“If you’re going out there as he is and saying ‘Trust Ted’ and ‘I am the guy you can trust all the time because I’m not going to waver’ [but] then you have a whole laundry list of wavers and changes, then I think it’s fair game.”

The issues where Cruz has allegedly shifted his position recently include: Free trade, the annual inflow of foreign workers, and ethanol.

Santorum said what bothers him the most “is not that [other candidates] shift, because there’s certain things that happen… but when someone says, ‘No, I never had that position’ and tries to deny what is obvious that they did… I would just say to voters be careful of people who are not being particularly forthright with you.”

“My message doesn’t change depending on what state or what group I’m in front of. I think that’s really one of the most important things to think about,” Santorum said, adding that:

You get all of these candidates that go to New York City or go to Los Angeles and get in front of this group or get in front of that group and they sort of, well lets just say, don’t have the level of enthusiasm and commitment in front of those groups as they do in front of a group that’s more at the base of our party. I’m a believer that if you’re a conservative and you really want to shake things up that means you go in front of every group and shake them up too.

“If you really want to win with something that’s meaningful… and make a difference, then you got to go out and be on almost like a crusade,” Santorum said. “It’s one thing to take on the establishment… when you’re in front of the establishment” and not a group of grassroots conservatives and Tea Party supporters.

Santorum stressed the need for conservatives to select a candidate that has a proven track record of winning.

“If you look at the top and the bottom of the tickets, the higher you are, the less experience you have. And the lower you are, the more experience you have. Experience is actually a negative in this election,” Santorum argued.

Santorum explained that as a former senator, he’s been able to reach out to the “blue collar party” and represent the steel mills in Pennsylvania.

“If we’re going to be successful nationally, then we better start winning states like, Pennsylvania, Michigan, Ohio, Wisconsin,” Santorum warned. “It’s who our base is in the future.”